Vascular tumors of the ovary are very rare. We report a case of ovarian hemangioma in a patient treated with tamoxifen for breast ductal carcinoma. CD31 and CD34 immunoreactivity confirmed the vascular origin of the tumor. It is interesting to note that estrogen and progesterone receptors were negative in endothelial cells of the hemangioma, but were positive in stromal ovarian cells. Tamoxifen is a synthetic, non-steroidal, anti-estrogenic drug widely used as adjuvant therapy for pre- and post-menopausal, early and metastatic, breast cancer patients with positive estrogen receptor proteins. The mechanism of action of tamoxifen in stimulating the development and/or growth of ovarian hemangioma is unknown. We may speculate that its prolonged, estrogenic effect on the ovary may be one of the stimulating factor.